CHICAGO U.S. lawmakers are investigating whether U.S. Environmental Protection Agency staff influenced the World Health Organization's review of glyphosate and its finding that the herbicide probably causes cancer, according to a letter sent to the agency on Tuesday.

The letter from the U.S. House of Representatives Committee on Science, Space and Technology is part of an ongoing investigation into the agency after the EPA posted and withdrew an internal report that said glyphosate was not cancer causing.

One question that lawmakers are trying to answer, according to legislative sources, is whether EPA staff allowed personal bias to colour the agency's scientific review of glyphosate, the chemical in Monsanto Co's (MON.N) Roundup herbicide.

Some EPA staff participated in both the U.S. review and the WHO review. While the committee's line of investigation was not clear, lawmakers cited the contradictory findings of the two reviews.

Those reviews were by the EPA's cancer assessment review committee or CARC and the WHO's cancer arm, the International Agency for Research on Cancer or IARC.

“Given the apparent contradictions of the CARC and IARC findings for glyphosate ..., the committee has concerns about the integrity" of the WHO review, the role of EPA officials in that review and their influence on the outcome of the EPA study, the committee's letter to the EPA seen by Reuters said.

According to the letter, lawmakers want congressional staff to interview four top EPA officials who were involved in one or both reviews of glyphosate.

An EPA spokeswoman said Tuesday the agency had received the letter, was reviewing it and would respond.

In an earlier letter EPA sent to the committee, the agency said that publishing the cancer assessment review committee's report was an accident and that the cancer review was still ongoing.

The EPA said it was "currently reviewing our standard operating procedures for the release of documents to avoid the inadvertent release of pre-decisional information in the future."

The congressional committee began its investigation into the EPA last month after the report by the EPA's cancer assessment review committee became briefly public.

The report found that glyphosate was "not likely" to be carcinogenic to humans. It also appeared to dispute the IARC report and questioned its analysis.

The WHO's IARC last year classified glyphosate as "probably carcinogenic to humans."

The House's Agriculture Committee previously said it too was examining the agency's review of glyphosate and atrazine, another chemical used in agricultural herbicides.

The Agriculture Committee also wanted to know what steps still needed to be taken to finalise and issue the glyphosate report, which it had expected in July 2015.